I've been arguing again at WUWT. Lord Monckton has been writing a series of articles (see also here, here and here), with more promised, on feedbacks and what he calls the "official equation". Mentioning feedbacks brings out all the engineers talking about feedback and instability. In the course of arguing, I think I see some general confusions arising from inadequate specifications of what circuitry is envisaged, and other general unsoundness. So I thought I could try to clear that up here, and in the process show an actual circuit which would implement the "official equation".
The first thing I try to emphasise here is that climate is not a circuit, and feedbacks are not used in GCM's. They are not the basis of climate science; in fact climate scientists talk about them far less than people imagine. Feedbacks are diagnostic tools - inferred from model output (or climate data) to help understanding. And you are free to imagine any kind of circuit or other apparatus that you think helps. That is a starting point - people are not always imagining the same thing, but they use the same vocabulary.
The basic concept here is climate sensitivity (CS). If you add a warming heat flux, usually from greenhouse effect, how much will the temperature rise? To make this more definite, it is often expressed as equilibrium CS (ECS). If you add a flux and then keep it constant, how much will temperature have risen by the time it has settled to steady state?
A starting point is what can be called Planck sensitivity. We know from the Stefan-Boltzmann law that a warmer planet will radiate more heat, and that will give a relation between flux and temperature change. For a black body, flux F=σT4 (S-B, with σ as the S-B constant). I propose to make F analogous to current and T to voltage, so this gives
CS = dT/dF = 1/(4σT3) K/(W/m2)
This gives it the units of resistance (analogy). With T=255 K, the effective radiating temperature of Earth, that would be 0.26. Earth is not a simple body - the atmosphere has an effect, and GCMs say that the right figure is about 0.31 (Soden and Held, 2006).
But ECS is generally reckoned to be a lot higher, because of the effects of positive feedbacks, especially water vapor feedback. Discussions of these are in S&H just cited, or Roe 2009. So then come the claims that positive feedback is necessarily unstable. It isn't, because it in effect adds to the negative Planck feedback. But if it outweighed that (and any other negative feedbacks) then it would be. That is the basis for talk of tipping points and thermal runaway.
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